Cryogenic Simulation and Freezing Point Evaluation in Nitrogen Rejection from Natural Gas: A Coupled Aspen HYSYS–ThermoFAST Approach

Jafari, Mostafa; Vatani, Ali; Banakar, Mohammad

doi:10.22050/ijogst.2026.547495.1752

Cryogenic Simulation and Freezing Point Evaluation in Nitrogen Rejection from Natural Gas: A Coupled Aspen HYSYS–ThermoFAST Approach

Document Type : Research Paper

Authors

Mostafa Jafari ¹

Ali Vatani ²

Mohammad Banakar ³

¹ PhD Graduate and Researcher, School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran

² Professor, Institute of Liquefied Natural Gas (I-LNG), School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran

³ Department Manager, Engineering and Development Department, Artin Energy Pooya Company, Tehran, Iran

https://doi.org/10.22050/ijogst.2026.547495.1752

Abstract

Cryogenic nitrogen rejection from methane-rich natural gas is a critical operation for meeting LNG and pipeline gas specifications. However, it is highly susceptible to operational disturbances caused by solid formation in heat exchangers and distillation columns. This study proposes an integrated simulation framework that couples steady-state process modeling in Aspen HYSYS with rigorous solid–liquid–vapor equilibrium analysis in ThermoFAST to systematically evaluate freezing risks in CH4–N2 mixtures under nitrogen-rejection unit conditions. The nitrogen-rejection unit process, including multi-stream heat exchange, Joule–Thomson expansion, and cryogenic distillation, is modeled using the Peng–Robinson equation of state. ThermoFAST employs a Helmholtz-energy-based PC-SAFT equation of state, together with Lennard-Jones Weeks–Chandler–Andersen pure-solid references, to generate freezing envelopes over a wide pressure range of 0.1–10 MPa and across methane-rich to nitrogen-rich compositions relevant to industrial operations.
The framework is validated against available experimental solid–liquid equilibrium data, yielding mean absolute and relative deviations of 3.45 K and 5%, respectively, thereby demonstrating its suitability for hazard screening applications. Simulation results indicate that increasing pressure elevates the eutectic temperature and expands the stability region of the solid phase. In contrast, increasing nitrogen concentration depresses the eutectic temperature and narrows the solid stability range. Risk maps show that solid formation is most probable in methane-rich streams with CH4 greater than 0.75 at pressures of 10 MPa or higher, as well as in nitrogen-rich streams with CH4 less than 0.55 at extremely low temperatures, particularly after expansion and on the upper trays of the distillation column. The proposed integrated approach provides a predictive tool for identifying vulnerable operating zones, defining safe temperature–pressure margins, and enhancing the safety, operability, and efficiency of cryogenic nitrogen rejection processes.

Highlights

· Modeling of the smart water flooding process with polymer.

· Laboratory investigation and simulation to improve understanding of the effective mechanisms governing polymer-assisted smart water injection.

· Chemical enhanced oil recovery.

· Polymer–smart water flooding demonstrated a high potential for wettability alteration.

· Polymer–smart water flooding exhibited significant potential to enhance sweep efficiency.

Keywords

Aspen HYSYS–ThermoFAST coupling, Cryogenic nitrogen rejection (NRU), Freezing risk assessment, PC-SAFT (Helmholtz-energy EOS), Solid–liquid–vapor equilibrium (SLVE)

Subjects

Chemical Engineering – Gas Processing and Transmission

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